When stress is applied to a system in man-made materials failure of the
material occurs shortly after the initial defect forms. There are several
applications where the ability to prevent or postpone failure after the initial
defect would be extremely useful. Currently there are only a very limited
amount of materials which exhibit these self-healing characteristics. Two
examples are SiC [1] and MoSi2 heating elements, which form a
protective silica coating on their surface after being scratched. However, this
process occurs only at extremely high temperatures and is thus not suitable in
many applications.

To increase the life span of materials following damage new methods
involving electrohydrodynamics are being explored. This technology is being
tested initially with a system that contains two cylinders, one inside the
other, with an electrical current applied to the system as shown in the
following schematic. Between the two cylinders is a colloidal dispersion of
polystyrene particles, which are used to repair defects that occur in the inner
cylinder. When a defect occurs the current density at the damaged site is
increased, causing colloidal particles to coagulate around the defect
[2].

A schematic for a self-healing system that uses the electrohydrodynamic coagulation of
particles to close a defect in a cylinder wall.

The defect is not completely repaired because the voids between
the colloids prevent the formation of a completely dense surface. To fill in
these interstitial spaces various salts are also dissolved in the colloidal
dispersion (e.g., NiSO4) and these electrochemically deposit within
the voidal spaces ultimately filling the void [3]. This process produces a
coating of a ceramic/metal composite at the site of the initial defect.

Optical microscope images of
colloidal particles (2Ám polystyrene) forming on an ITO coating on a glass
substrate. (Left) A scratch that does not penetrate to the substrate encourages
the preferential coagulation of particles at the scratch. (RIght) However, a
penetrating crack prevents particle coagulation even under conditions favoring
the formation of 2-dimensional crystalline layers of particles. (Micrographs
courtesy of William Ristenpart)